1. Field of the Invention
The present invention relates to methods and scatterometers usable, for example, in the manufacture of devices by lithographic techniques.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Scatterometers may be used to measure several different embodiments of lithographic apparatuses, including their positioning errors of the substrate prior to exposure and exposure efficacy. Two important parameters of a lithographic apparatus (and specifically of the exposure action that the lithographic apparatus carries out) that may also be measured by scatterometers are focus and dose. A lithographic apparatus has an exposure apparatus that includes a radiation source and a projection system as mentioned below. The radiation source provides a beam of radiation and the projection system focuses the beam of radiation and applies a pattern to the beam to create a patterned beam of radiation that strikes the resist on the substrate surface. The dose of radiation that is projected onto a substrate in order to expose it, is controlled by various parts of the exposure apparatus. It is mostly the projection system of the lithographic apparatus that is responsible for the focus of the radiation onto the correct portions of the substrate. It is important that the focusing of the image of the pattern in the patterned radiation occurs at the surface of the substrate where the exposure occurs. This is so that the sharpest (i.e., most focused) image will occur on the surface of the substrate and the sharpest pattern possible may be exposed thereon. This enables smaller product patterns to be printed.
The focus and dose of the radiation directly affect various parameters of the patterns or structures that are exposed on the substrate. Parameters that can be measured using a scatterometer are physical properties of structures within the patterns that have been printed onto a substrate. These parameters may include the critical dimension (CD) or sidewall angle (SWA). The critical dimension is effectively the mean width of a structure such as a bar (or a space, dot or hole, depending on what the measured structures are that are in the printed pattern). The sidewall angle is the angle between the surface of the substrate and part of the rising (or falling) portion of the structure.
In addition, mask shape corrections (focus corrections for correcting for the bending of a mask) can be applied if scribe lane structures are used with a product mask for focus measurements.
It is desirable to provide a method of measuring focus using a scatterometer wherein the size of the target may be made smaller than the radiation beam spot.
Smaller markers for positioning, overlay- and CD-metrology, and focus dose metrology reduce real estate consumption for metrology. Smaller targets are more sensitive for etch process micro-loading and other process effects like non-conformal deposition and chemical and mechanical polishing. The complex processes of lithography and especially etch processes such as reactive-ion-etching (RIE) or plasma etching result for example in a (product) environment dependency of the etch rate (etch proximity). These micro-loading and process effects at (sub-)micrometer scale are undesirable for the production of semiconductor devices, and may perturb metrology on small targets differently than product features or differently over the width of the target. Particularly non-uniformity at the target-edge can cause metrology problems for overfill illumination, where the detection beam is larger than the target, combined with pupil-detection in optical metrology.
Micro-loading and process effects on metrology targets are difficult to detect because it concerns properties that occur within the processed layers of a wafer, for example the local etch rate for the bottom-grating of an overlay diffraction grating.
Detection of such micro-loading and process effects on metrology targets requires the application of an additional measurement technique such as scanning electron microscopy (SEM) or optical microscopy. However, these techniques have a limited sensitivity with respect to profile asymmetries of measured structures. Specific disadvantages of optical microscopy and top-down SEM are:
i. they are an additional “inspection” measurement;
ii. they need in most cases a different measurement tool than the actual metrology measurement using the inspected metrology targets;
iii. they give only limited information about profile asymmetries and their variations within the metrology target; and
iv. they cannot be used to improve the measurement of the actual target, they can only help for deciding if a metrology target can be used or not for a measurement.
Other methods to detect and study micro-loading and process-effects are transmission electron microscopy (TEM) and cross-section SEM. These have access to the profile information of the structured layers. However, both are time-consuming, destructive techniques because the wafer has to be cut along a line at the structure of interest for the cross-sectional view. Furthermore, only a single local cross-section can be prepared; TEM and cross-section SEM do not allow for extraction of 2-dimensional information locally over the wafer field.
Scanning Probe Microscopy (SPM) techniques such as Atomic Force Microscopy (AFM) on the freshly etched structure without top-layers is another possible inspection technique. However the technique is rather slow and it interrupts the production of wafers. The measurement is furthermore performed at an unfinished target, while one would like to know the effects in the complete layer structure.
Also, diffraction-based reconstruction via pupil detection may be a candidate technique to observe process effects. However, diffraction-based pupil detection combined with reconstruction is only able to probe process-effects on a large scale of the order of the illumination spot size (tens of microns). For pupil detection, the local information at sub-micron scale is hardly accessible (unless entire targets or structures are completely reconstructed, using for example an electro-magnetic solver in a recurrent solving loop, however that requires an unfeasible number of fit parameters describing e.g., the side-wall-angle of each individual line in the grating structure). Furthermore, the reconstruction necessary to retrieve the inspection information is time-expensive.
It is desirable to provide a method of detecting such micro-loading and process effects on metrology targets.